Rechargeable aluminum ion battery

ABSTRACT

A rechargeable battery using a solution of an aluminum salt as an electrolyte is disclosed, as well as methods of making the battery and methods of using the battery.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/290,599, filed Oct. 11, 2016, which claims benefit under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 62/238,935, filed Oct. 8,2015, the contents of both of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to rechargeable batteries using chargecarriers comprising aluminum ions, and particularly batteries withreduced toxic components to minimize human health hazards andenvironmental damage.

Description of the Background

With an increase in interest in generating electricity from renewableenergy sources such as wind and solar, it has become increasinglyimportant to identify a viable battery storage system. Lead acidbatteries, for instance, are the most widely used battery technology forgrid storage owing to their low cost (about $100-$150/kWh). However,lead acid batteries have a comparatively low gravimetric energy density(30-50 Wh/kg) and a poor cycle life, between 500 and 1000charge/discharge cycles, based on the low depths of discharge (50-75%).In addition, lead acid batteries have significant safety problemsassociated with handling and disposal, due to the presence of sulfuricacid and toxic lead components. Reports of increased lead poisoning andacid-related injuries among workers and children exposed to unsafehandling and disposal of lead acid batteries, have raised strongconcerns over large-scale implementation of lead acid batteries asstorage for electricity generated from renewable energy sources.

The search for economical alternatives for electrical storage that lackthe environmental and health risks of lead acid batteries has not beensuccessful. One alternative, sodium ion batteries, are estimated toreach a price of about $250/kWh by 2020, but the volumetric energydensity of sodium ion battery technology is lower than that of lead acidbatteries at less than about 30 Wh/L.

Another alternative, vanadium redox flow batteries, offer high capacity,long discharge times and high cycle life, but have relatively lowgravimetric and volumetric energy densities, and are expensive due tothe high cost of vanadium and other components. Liquid metal batterieson the other hand are based on ion exchange between two immisciblemolten salt electrolytes, but must operate at high temperatures, up to450° C., rely on a complicated lead-antimony-lithium composite for ionexchange, and such systems have problems of flammability and toxicity.

SUMMARY OF THE INVENTION

A rechargeable battery using an electrolyte comprising aluminum ions isdisclosed, as well as methods of making the battery and methods of usingthe battery.

In certain embodiments, a battery is disclosed that includes an anodecomprising aluminum, an aluminum alloy or an aluminum compound, acathode, a porous separator comprising an electrically insulatingmaterial that prevents direct contact of the anode and the cathode, andan electrolyte comprising a solution of an aluminum salt, wherein theelectrolyte is in electrical contact with the anode and the cathode. Inpreferred embodiments, the battery is a rechargeable battery, that is, asecondary battery.

In certain embodiments, the anode is an aluminum alloy comprisingaluminum and at least one element selected from the group consisting ofmanganese, magnesium, lithium, zirconia, iron, cobalt, tungsten,vanadium, nickel, copper, silicon, chromium, titanium, tin and zinc. Incertain embodiments, the anode is aluminum that has received a treatmentthat is effective to increase the hydrophilic properties of the anodesurface that is in contact with the electrolyte. In certain embodiments,the surface treatment comprises the step of contacting a surface of thealuminum with an aqueous solution of an alkali metal hydroxide.Typically, the alkali metal hydroxide selected from the group consistingof lithium hydroxide, sodium hydroxide, potassium hydroxide and mixturesthereof. In certain embodiments, anode is an aluminum metal foil or analuminum alloy foil.

In certain embodiments, the anode is an aluminum compound selected fromthe group consisting of an aluminum transition metal oxide(Al_(x)M_(y)O_(z), where M is a transition metal selected from the groupconsisting of iron, vanadium, titanium, molybdenum, copper, nickel,zinc, tungsten, manganese, chromium, cobalt and mixtures thereof and x,y, and z range from 0 to 8, inclusive); an aluminum transition metalsulfide, (Al_(x)M_(y)S_(z), where M is a transition metal selected fromthe group consisting of iron, vanadium, titanium, molybdenum, copper,nickel, zinc, tungsten, manganese, chromium, cobalt and mixtures thereofand x, y, and z range from 0 to 8, inclusive); aluminum lithium cobaltoxide (AlLi₃CoO₂); lithium aluminum hydride (LiAlH₄); sodium aluminumhydride (NaAlH₄); potassium aluminum fluoride (KAlF₄); and mixturesthereof.

In certain embodiments, the electrolyte is an aqueous solution of analuminum salt selected from the group consisting of aluminum nitrate,aluminum sulfate, aluminum phosphate, aluminum bromide hexahydrate,aluminum fluoride, aluminum fluoride trihydrate, aluminum iodidehexahydrate, aluminum perchlorate, aluminum hydroxide, and combinationsthereof. In certain embodiments, the molarity of the aluminum saltranges from 0.05 M to 5 M and the concentration of water ranges from 5weight % to 95 weight %.

In certain embodiments, the electrolyte further comprises an alkalimetal hydroxide selected from the group consisting of lithium hydroxide,sodium hydroxide, potassium hydroxide, ammonium hydroxide, calciumhydroxide, magnesium hydroxide and mixtures thereof. In certainembodiments, the electrolyte comprises an aqueous solution of aluminumnitrate and lithium hydroxide in a molar ratio of about 1:1 to about1:10. In certain embodiments, the electrolyte comprises a polymerselected from the group consisting of polytetrafluoroethylene,acetonitrile butadiene styrene, styrene butadiene rubber, ethyl vinylacetate, poly(vinylidene fluoride-co-hexafluoropropylene), polymethylmethacrylate, and mixtures thereof.

In certain embodiments, the electrolyte comprises an aluminum halideselected from the group consisting of aluminum chloride, aluminumbromide, aluminum iodide, and mixtures thereof and a 1-ethylmethylimidazolium halide selected from the group consisting of 1-ethylmethylimidazolium chloride, 1-ethyl methylimidazolium bromide, 1-ethylmethylimidazolium iodide and mixtures thereof. In certain embodiments,the aluminum halide and the 1-ethyl methylimidazolium halide are presentin the ratio of 1:1 to 5:1 (weight:weight).

In certain embodiments, the electrolyte comprises a solvent selectedfrom the group consisting of water, ethanol, N-methyl pyrrolidone,dimethyl sulfoxide and mixtures thereof.

In certain embodiments, the cathode comprises a material selected fromthe group consisting of lithium manganese oxide, acid-treated lithiummanganese oxide, lithium metal manganese oxide (where the metal isselected from the group consisting of nickel, cobalt, aluminum, chromiumand combinations thereof), acid-treated lithium metal manganese oxide,graphite metal composite (where the metal is an electrically conductivemetal selected from the group consisting of nickel, iron, copper,cobalt, chromium, aluminum and mixtures thereof), graphite-graphiteoxide, manganese dioxide and graphene. In certain preferred embodiments,cathodes comprising lithium have been subjected to acid treatment. Incertain preferred embodiments, the anode comprises aluminum metal, thecathode comprises graphite-graphite oxide, and the aluminum saltcomprises aluminum nitrate. In other preferred embodiments, the anodecomprises aluminum metal, the cathode comprises acid-treated lithiummanganese oxide, and the aluminum salt comprises aluminum nitrate.

In certain embodiments, the porous separator comprises a materialselected from the group consisting of polyethylene,polytetrafluoroethylene, polyvinyl chloride, ceramic, polyester, rubber,polyolefins, glass mat, polypropylene, a mixed cellulose ester, nylon,glass microfiber and mixtures and combinations thereof. The separatorsmay be treated with or mixed with hydrophilic functional groups,monomers or polymers, including but not limited to acrylic acid,diethyleneglycol-dimethacrylate, cellulose acetate and silicon oxide, inorder to introduce hydrophilicity for use with aqueous electrolytes. Incertain embodiments, the porous separator has an average pore size ofabout 0.067 μm to about 1.2 μm.

In certain embodiments, a battery is disclosed that comprises an anodecomprising aluminum, an aluminum alloy comprising aluminum and at leastone element selected from the group consisting of manganese, magnesium,lithium, zirconia, iron, cobalt, tungsten, vanadium, nickel, copper,silicon, chromium, titanium, tin and zinc; an aluminum compound selectedfrom the group consisting of an aluminum transition metal oxide(Al_(x)M_(y)O_(z), where M is a transition metal selected from the groupconsisting of iron, vanadium, titanium, molybdenum, copper, nickel,zinc, tungsten, manganese, chromium, cobalt and mixtures thereof and x,y, and z range from 0 to 8, inclusive); an aluminum transition metalsulfide, (Al_(x)M_(y)S_(z), where M is a transition metal selected fromthe group consisting of iron, vanadium, titanium, molybdenum, copper,nickel, zinc, tungsten, manganese, chromium, cobalt and mixtures thereofand x, y, and z range from 0 to 8, inclusive); aluminum lithium cobaltoxide (AlLi₃CoO₂); lithium aluminum hydride (LiAlH₄); sodium aluminumhydride (NaAlH₄); potassium aluminum fluoride (KAlF₄); or mixturesthereof; a cathode comprising a material selected from the groupconsisting of lithium manganese oxide, acid-treated lithium manganeseoxide, lithium metal manganese oxide (where the metal is selected fromthe group consisting of nickel, cobalt, aluminum, chromium andcombinations thereof), acid-treated lithium metal manganese oxide,graphite metal composite (where the metal is an electrically conductivemetal selected from the group consisting of nickel, iron, copper,cobalt, chromium, aluminum and mixtures thereof), graphite-graphiteoxide, manganese dioxide and graphene; and an electrolyte comprising anaqueous solution of an aluminum salt selected from the group consistingof aluminum nitrate, aluminum sulfate, aluminum phosphate, aluminumbromide hexahydrate, aluminum fluoride, aluminum fluoride trihydrate,aluminum iodide hexahydrate, aluminum perchlorate, aluminum hydroxide,and combinations thereof. In certain embodiments, the molarity of thealuminum salt ranges from 0.05 M to 5 M and the concentration of waterranges from 5 weight % to 95 weight %. Typically, the battery furthercomprises a porous separator comprising an electrically insulatingmaterial that prevents direct contact of the anode and the cathode. Incertain embodiments, the porous separator comprises a material selectedfrom the group consisting of polyethylene, polytetrafluoroethylene,polyvinyl chloride, ceramic, polyester, rubber, polyolefins, glass mat,polypropylene, a mixed cellulose ester, nylon, glass microfiber andmixtures and combinations thereof. The separators maybe treated with ormixed with hydrophilic functional groups, monomers or polymers,including but not limited to acrylic acid,diethyleneglycol-dimethacrylate, cellulose acetate and silicon oxide, inorder to introduce hydrophilicity for use with aqueous electrolytes. Incertain embodiments, the porous separator has an average pore size ofabout 0.067 μm to about 1.2 μm.

Also disclosed are methods of using a rechargeable battery that includesan anode comprising aluminum, an aluminum alloy or an aluminum compound,a cathode, a porous separator comprising an electrically insulatingmaterial that prevents direct contact of the anode and the cathode andan electrolyte comprising an aqueous solution of an aluminum salt. Incertain embodiments, a system is disclosed that includes at least onesuch rechargeable battery that is operatively connected to a controller,wherein the controller is adapted to be operatively connected to asource of electrical power and to a load. In certain embodiments, thecontroller is effective to control the charging of the battery by thesource of electrical power. In certain embodiments, the controller iseffective to control the discharging of the battery by the load. Incertain embodiments, the controller is adapted to provide a dischargepattern that is a combination of high and low current densitygalvanostatic steps. In certain embodiments, the source of electricalpower is a solar panel or wind-powered generator. In certainembodiments, the load is a local electrical load or a power distributiongrid.

The above described and other features are exemplified by the followingfigures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will be apparent fromthe following more particular description of exemplary embodiments ofthe disclosure, as illustrated in the accompanying drawings, in whichlike reference characters refer to the same parts throughout thedifferent views. The drawings are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the disclosure.

FIG. 1A is a photograph of aluminum foil suitable for use as an anodethat has been treated with a drop of an aqueous solution of lithiumhydroxide; FIG. 1B is a photograph of the piece of the treated aluminumfoil of FIG. 1A showing a change in the appearance of the drop of theaqueous solution of lithium hydroxide; FIG. 1C is a photograph of thepiece of treated aluminum foil of FIG. 1B showing the greyish-whiteappearance of the aluminum foil following the drying of the lithiumhydroxide solution; FIG. 1D is a photograph of the piece of treatedaluminum foil of FIG. 1C showing the effect of placing a drop ofdeionized water on the treated aluminum foil indicating an increase inhydrophilicity of the treated aluminum foil; and FIG. 1E is a photographof a drop of deionized water on untreated aluminum foil for comparisonto FIG. 1D.

FIG. 2 is a schematic diagram of an exploded view of a test battery 100in a coin cell format, showing the positive case 110, a spring 120, afirst spacer 130, the cathode 140, the separator 150, the anode 160, asecond spacer 170 and the negative case 180.

FIG. 3 is an x-ray photoelectron spectroscopy (XPS) profile of carbonsheets following a 100% depth of discharge, showing a strong peakcorresponding to Al 2p transition, associated with the presence ofgibbsite, Al(OH)₃, crystals.

FIG. 4A shows the voltage profile that was produced by applying currentat a current density of 0.1 mA/cm² to a test battery having an anodecomprising an aluminum foil treated with LiOH as described in Example 1,a cathode comprising acid-treated lithium manganese oxide, a 25 μm thickpolypropylene separator with an average pore size of 0.067 μm, and a 0.5M aqueous aluminum nitrate electrolyte. FIG. 4B shows the voltageprofile that was produced by applying current at a current density of0.1 mA/cm² to a test battery having an anode comprising an aluminum foiltreated with LiOH as described in Example 1, a cathode comprisinggraphite-graphite oxide, a 25 μm thick polypropylene separator with anaverage pore size of 0.067 μm, and a 0.5 M aqueous aluminum nitrateelectrolyte. The observed average operating voltage is significantlyhigher with the use of acid-treated lithium manganese oxide cathodes,possibly owing to the higher activation energy for diffusion andintercalation of ions. Carbon is known to possess a sufficiently lowactivation energy for diffusion and intercalation of metal ions (theintercalation voltage of lithium ions in carbon against a lithium metaloccurs at about 100 mV).

FIG. 5A shows the battery capacity as a function of cycle index of abattery having an anode comprising an aluminum foil treated with LiOH asdescribed in Example 1, a 25 μm thick polypropylene separator with anaverage pore size of 0.067 μm, and a 0.5 M aqueous aluminum nitrateelectrolyte and a graphite-graphite oxide composite cathode. Thecoulombic efficiency was estimated to be close to 100%, indicatingefficient and reversible charge and discharge kinetics. FIG. 5B showsthe battery capacity as a function of cycle index of a battery having ananode comprising an aluminum foil treated with LiOH as described inExample 1, a 25 μm thick polypropylene separator with an average poresize of 0.067 μm, and a 0.5 M aqueous aluminum nitrate electrolyte andan acid-treated Li_(1-x)MnO₂ cathode. The reduction in capacity afterover 800 charge/discharge cycles is only about 3% of the originalcapacity.

FIG. 6A shows sequential cyclic voltammetry profiles of a battery havingan anode comprising an aluminum foil treated with LiOH as described inExample 1, a 25 μm thick polypropylene separator with an average poresize of 0.067 μm, and a 0.5 M aqueous aluminum nitrate electrolyte andan acid-treated lithium manganese oxide cathode. FIG. 6B showssequential cyclic voltammetry profiles of a battery having an anodecomprising an aluminum foil treated with LiOH as described in Example 1,a 25 μm thick polypropylene separator with an average pore size of 0.067μm, and a 0.5 M aqueous aluminum nitrate electrolyte and agraphite-graphite oxide cathode. The test batteries in the coin cellformat were cycled at various voltage sweep rates between 10 mV/sec and50 mV/sec within a voltage range of 0 V and 1.5 V.

FIG. 7A shows the results of electrochemical impedance spectroscopy(EIS) of a test cell having an aluminum anode and an acid-treatedlithium manganese oxide cathode. FIG. 7B shows the results ofelectrochemical impedance spectroscopy (EIS) of a test cell having analuminum anode and a graphite-graphite oxide cathode. Insets show theRandles equivalent circuit used to fit the spectra.

FIG. 8A is a schematic representation of a prismatic cell 80.

FIG. 8B illustrates the discharge voltage profile of a prismatic cellrated at 1 mAh. The cell had an anode comprising an aluminum foiltreated with LiOH as described in Example 1, a cathode comprisingacid-treated lithium manganese oxide, a 25 μm thick polypropyleneseparator with an average pore size of 0.067 μm, a 0.5 M aqueousaluminum nitrate electrolyte and were tested at 10 μA/cm².

FIG. 9A is a schematic representation of a pouch cell 90.

FIG. 9B illustrates the discharge profile of a pouch cell comprising 0.8cm×1 cm electrodes and hydrophilic polypropylene separators. The cellhad an anode comprising an aluminum foil treated with LiOH as describedin Example 1, a cathode comprising acid-treated lithium manganese oxide,a 25 μm thick polypropylene separator with an average pore size of 0.067μm, a 0.5 M aqueous aluminum nitrate electrolyte and were tested atabout 25 μA/cm².

FIG. 10 is a block diagram of a system 800 that incorporates the battery810 of the present disclosure, showing a controller 820 that isoperatively connected to battery 810, a source of electrical power 830,a local electrical load 840 and an electrical power distribution grid850.

FIG. 11 compares the discharge and charging properties of two batteriesdiffering in electrolyte composition: one battery having a 0.5 MAl(NO₃)₃ (aq) electrolyte (curve 1) and another battery having a 0.5 MAl(NO₃)₃ and 2 M LiOH (aq) electrolyte (curve 2). Each battery wasassembled in a 2032 coin cell format and had an anode comprising analuminum foil treated with LiOH as described in Example 1, a cathodecomprising acid-treated lithium manganese oxide, a 25 pin thickpolypropylene separator with an average pore size of 0.067 μm, and wastested at current densities of 10 μA/cm².

FIG. 12 illustrates the effect of separator pore size on the averagedischarge potential produced at a given current density, where pentagons(1) represent measurements made on a battery having a polypropyleneseparator with 0.067 μm pores, a triangle (2) represents measurementsmade on a battery having a mixed cellulose ester separator with 0.20 μmpores, a circle (3) represents measurements made on a battery having anylon separator with 0.45 μm pores, squares (4) represent measurementsmade on a battery having a nylon separator with 0.80 μm pores, anddiamonds (5) represent measurements made on a battery having a glassmicrofiber separator with 1.0 μm pores. Each battery was assembled in a2032 coin cell format and had an anode comprising an aluminum foiltreated with LiOH as described in Example 1, a cathode comprisingacid-treated lithium manganese oxide, and the electrolyte was an 0.5 Maqueous aluminum nitrate solution. Polypropylene separators (pentagons)were tested at 10 pA/cm² and 20 pA/cm²; mixed cellulose ester separators(triangle) and nylon separators (circle) were tested at 20 pA/cm²; nylonseparators (squares) were tested at 20 pA/cm², 40 pA/cm² and 50 pA/cm²;and glass microfiber separators (diamonds) were tested at 20 pA/cm² and40 pA/cm².

FIG. 13 illustrates the discharge of a battery having a polypropyleneseparator with 0.067 μm pores at a current density of 10 pA/cm². Thebattery was assembled in a 2032 coin cell format and had an anodecomprising an aluminum foil treated with LiOH as described in Example 1,a cathode comprising acid-treated lithium manganese oxide, and theelectrolyte was an 0.5 M aqueous aluminum nitrate solution.

FIG. 14 illustrates the discharge of a battery having a nylon separatorwith 0.80 μm pores at a current densities of 20 pA/cm² (curve 1), 40pA/cm² (curve 2), and 40 pA/cm² (curve 3). The battery was assembled ina 2032 coin cell format and had an anode comprising an aluminum foiltreated with LiOH as described in Example 1, a cathode comprisingacid-treated lithium manganese oxide, and the electrolyte was an 0.5 Maqueous aluminum nitrate solution.

FIG. 15 illustrates the discharge of a battery having a glass microfiberseparator with 1.0 μm pores at a current densities of 20 pA/cm²(curve 1) and 40 μA/cm² (curve 2). The battery was assembled in a 2032coin cell format and had an anode comprising an aluminum foil treatedwith LiOH as described in Example 1, a cathode comprising acid-treatedlithium manganese oxide, and the electrolyte was an 0.5 M aqueousaluminum nitrate solution.

FIG. 16 is a photograph of the free-standing, translucent solid polymerelectrolyte measuring about 1 mm in thickness and about 3 cm indiameter.

FIG. 17 illustrates the voltage profile of a battery having a solidpolymer electrolyte, showing a short duration of discharge at 50 pA/cm²,followed by discharging at 20 pA/cm² and charging at a current densityof 20 pA/cm², with an inset of a photograph of solid polymerelectrolytes. The battery was assembled in a 2032 coin cell format andhad an anode comprising an aluminum foil treated with LiOH as describedin Example 1, a cathode comprising acid-treated lithium manganese oxide,and a 25 μm thick polypropylene separator with an average pore size of0.067 μm. The surface of the cathode was further treated with 2 M LiOHprior to assembly and testing. The electrolyte was prepared by mixing6.7 weight % poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP), 20 weight % aluminum nitrate and 10 weight % LiOH in 73.3weight % deionized water. The solution was then placed inside a furnacemaintained at 120° C. overnight to remove the water content and obtainthe resultant solid polymer electrolyte.

FIG. 18 shows a discharge profile produced by a combination oflow-current and high-current pulses. The battery was assembled in a 2032coin cell format and had an anode comprising an aluminum foil treatedwith LiOH as described in Example 1, a cathode comprising acid-treatedlithium manganese oxide, a 0.5 M aluminum nitrate (aq) electrolyte and a25 μm thick polypropylene separator with an average pore size of 0.067μm. The current densities were switched between 100 A/g (low-currentpulse) and 500 A/g (high-current pulse), where the current is normalizedwith respect to the mass of the cathode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following non-limiting examples further illustrate the variousembodiments described herein.

The aluminum ion battery chemistry described in this disclosure relieson simple electrode and aqueous electrolyte chemistry and is based onthe movement of hydroxyaluminates between an aluminum anode source and ahost cathode material. Incorporation of aluminum anode and graphite oracid-treated lithium manganese oxide cathode, along with an aqueouselectrolyte comprising an inexpensive aluminum salt ensurescost-competitiveness of the technology. Aluminum is abundantly availableand is inherently safer and electrochemically more robust compared tolithium metal, facilitating the use in aqueous environments as well asambient atmospheric conditions in a safe and reliable manner. Moreover,the approach adopted here to incorporate hydrophilicity toaluminum-based anodes is inexpensive and highly scalable. Both carbonand lithium manganese oxide cathodes are easy to manufacture and areconsidered to be extremely safe over a wide range of operatingconditions and are compatible with a wide range of aqueous, non-aqueous,ionic and solid electrolytes, lending flexibility and scalability to thebattery technology. Moreover the use of air stable electrodes andaqueous electrolytes is expected to significantly reduce the time, costand complexity of manufacturing of the proposed aluminum ion aqueousbattery relative to other competing battery chemistries that rely onelaborate manufacturing and assembly techniques, often inhumidity-controlled dry room environments. The preliminary performanceparameters indicate excellent reliability and repeatability. Theestimated volumetric and gravimetric energy density are about 30 Wh/Land 75 Wh/kg respectively for the aluminum-graphite system and 50 Wh/Land 150 Wh/kg respectively for the aluminum-acid-treated lithiummanganese oxide system, normalized with respect to the mass and volumeof cathode, thereby offering significant advancements over alternateemerging battery technologies.

In general, multivalent ion transfer has been a holy grail for batterytechnology. In the commercially available lithium ion battery, anelectron removed from lithium metal travels through the external loadwhile a corresponding single equivalent charged ion is transferredbetween the anode and cathode of the battery through the electrolyte.For a long time, efforts to increase energy density have sought totransfer an equivalent ion between the electrodes when multipleelectrons are removed from an atom of a metal such as magnesium,calcium, zinc, aluminum, manganese, or vanadium. Such an approach hasthe potential to linearly scale energy density in proportion to thenumber of electrons removed from each atom. However, such efforts havenot been successful, as multivalent ions are inherently highly reactiveand cannot be successfully transported through the electrolyte to thecounter electrode. The present inventors have developed an innovativesolution to this challenging problem by transforming a multivalent ioninto a single monovalent ion which is much easier to transport to thecounter electrode. As disclosed herein, this concept is broadlyapplicable across various types of multivalent ions. In the disclosedembodiments, while three electrons are removed from an aluminum atom,instead of carrying a single ion with equivalent charge, the aluminumion (Al³⁺) combines with hydroxides to form a single ion with only asingle charge (Al(OH)₄ ¹⁻) that is transported to the counter electrode.While not wishing to be bound by theory, it is believed that thisapproach eliminates the problem associated with the highly reactivemultivalent ions and yet has the ability to linearly scale the energydensity when multiple electrons are removed from a metal. In otherembodiments, a multivalent ion could be transformed into more than onetype of monovalent ions subject to the chemistry deployed. In furtherembodiments, a multivalent ion, such as an ion of +3, can be transformedinto ions that are either monovalent or multivalent type having reducedvalence or combinations thereof.

As used herein, “aluminum” and “aluminium” are used interchangeably torefer to the same element. “Aluminum” is the preferred term that isused.

As used herein, “aluminum ion” includes polyatomic aluminum anions, suchas the hydroxyaluminate anion, Al(OH)₄ ¹⁻, the tetrachloroaluminate ion,AlCl₄ ¹⁻, the tetrahydroaluminate ion, AlH₄ ¹⁻, and thehexafluoroaluminate ion, AlF₆ ¹⁻.

As used herein, “delithiation” or “delithiated” refers to the removal oflithium from a compound including lithium, such as lithium manganeseoxide, including removal by chemical methods, such as acid treatment,electrochemical methods or a combination of chemical and electrochemicalmethods. The product of the delithiation of lithium manganese oxide canbe expressed as Li_(1-x)MnO₂, where x denotes the amount of lithiumremoved by the delithiation method. As the delithiation methodapproaches complete removal of lithium, x approaches 1, and the productis substantially MnO₂. In certain embodiments, the product issubstantially MnO₂.

The next generation of energy storage technology should therefore enableelimination of the aforementioned disadvantages while simultaneouslyfacilitating lower costs. A list of the desirable attributes have beenprovided in Table 1, below. In addition, the US Department of Energy hasalso specified four specific challenges to large-scale deployment ofenergy storage for grids: (1) cost-competitive technology, (2) validatedreliability and safety, (3) equitable regulatory environment, and (4)industry acceptance.

TABLE 1 An overview of the desired attributes of next generation batterysystems and their potential impacts Characteristic Impact High operatingvoltages Fewer cells in series; Lower costs of implementation; Abilityto integrate in a wide range of applications including consumerelectronics Characteristic voltage plateau Simplified battery managementsystems Simple electrode and electrolyte Ease of manufacturing;components Enhanced safety; Lower cost Room temperature operationImproved safety Minimal external accessories Easy maintenance;(insulation, cooling, pumps, Lower cost storage tanks, etc.)

The aluminum-ion battery storage technology is based on the movement ofaluminum ions between an anode and a cathode, through an aqueouselectrolyte and a separator that is permeable to the aluminum ions. Incertain embodiments, the aluminum ions are polyatomic aluminum anions.In certain embodiments, the separator is a polymeric material. A porous,at least partially hydrophilic polymer separator provides an insulatingseparation layer between the anode and cathode, thereby preventingpotential shorting between the two electrodes. In certain embodiments,the polymer separator is a polypropylene, cellulose ester or nylonseparator. The porosity of the separator is adapted to facilitate themovement of the aluminum ions between the anode and cathode.

Aluminum electrochemistry has several advantages over the other batterytechnologies that are available today. Aluminum has a theoretical energydensity of 1060 Wh/kg, compared to 406 Wh/kg of lithium ions, due to thepresence of three valence electrons in aluminum as compared to onevalence electron in lithium. Aluminum is the third most abundant element(after oxygen and silicon), and the most abundant metal available in theearth's crust (8.1 weight %), compared to lithium (0.0017 weight %),sodium (2.3 weight %) and vanadium (0.019 weight %), providing anopportunity to reduce material costs. Finally, aluminum is bothmechanically and electrochemically robust and can be safely operated inambient air as well as humid environments while simultaneouslyfacilitating a greater flexibility in the choice of electrolytes(aqueous, organic, ionic and solid) and operating conditions.

While aluminum is the preferred element, other electrochemically activeelements that form hydroxides that possess sufficient ionic mobility andelectrical conductivity may also be used. Such elements include alkalimetals such as lithium, sodium and potassium, alkaline earth metals suchas calcium and magnesium, transition metals such as manganese, andpost-transition metals such as tin.

In certain embodiments, the electrolyte comprises an aqueous solution ofan aluminum salt. A preferred solvent is deionized water. In certainembodiments, the aluminum salts include aluminum nitrate, aluminumsulfate, aluminum phosphate, aluminum bromide hexahydrate, aluminumfluoride, aluminum fluoride trihydrate, aluminum iodide hexahydrate,aluminum perchlorate, aluminum hydroxide, and combinations thereof.Preferred aluminum salts are aluminum nitrate, aluminum bromidehexahydrate, aluminum fluoride, aluminum iodide hexahydrate, andcombinations thereof. In an embodiment, the aluminum salt is aluminumnitrate.

In certain embodiments, the aluminum salt is present in an aqueoussolution of about 0.05 M to about 5.0 M. In some embodiments, thealuminum salt is present in an aqueous solution of about 0.5 M to about3.0 M. In certain embodiments, the electrolyte comprises about 0.1M toabout 3.0 M sodium nitrate aqueous solution. In certain preferredembodiments, the electrolyte comprises about 1M to about 3.0 M sodiumnitrate (aqueous).

One of ordinary skill would recognize that the aqueous aluminum saltelectrolyte is environmentally benign, non-toxic and non-flammable andis therefore safer than organic electrolytes used in commercial lithiumion batteries and many sodium ion batteries today.

In certain embodiments, an anode comprises aluminum metal foils. Inpreferred embodiments the aluminum metal foil has been treated toincrease its hydrophilic properties. In other embodiments, an anodecomprises an aluminum compound selected from the group consisting of analuminum transition metal oxide (Al_(x)M_(y)O_(z), where M is atransition metal selected from the group consisting of iron, vanadium,titanium, molybdenum, copper, nickel, zinc, tungsten, manganese,chromium, cobalt and mixtures thereof and x, y, and z range from 0 to 8,inclusive); an aluminum transition metal sulfide, (Al_(x)M_(y)S_(z),where M is a transition metal selected from the group consisting ofiron, vanadium, titanium, molybdenum, copper, nickel, zinc, tungsten,manganese, chromium, cobalt and mixtures thereof and x, y, and z rangefrom 0 to 8, inclusive); aluminum lithium cobalt oxide (AlLi₃CoO₂);lithium aluminum hydride (LiAlH₄); sodium aluminum hydride (NaAlH₄);potassium aluminum fluoride (KAlF₄); and mixtures thereof.

In certain embodiments, an anode comprises an alloy of aluminum and oneor more metals selected from the group consisting of lithium, sodium,potassium, manganese and magnesium and combinations thereof. In certainembodiments, the anode comprises an aluminum alloy comprising aluminumand at least one element selected from the group consisting ofmanganese, magnesium, lithium, zirconia, iron, cobalt, tungsten,vanadium, nickel, copper, silicon, chromium, titanium, tin, zinc andcombinations thereof. The operating voltage in batteries that usecarbon-based cathodes could be increased through the incorporation ofhigh activation energy alloy anodes.

Improvement of cathode materials can be gained by introduction ofporosity and voids or modifications in the grain structure andorientation within the existing graphite or acid-treated lithiummanganese oxide compositions. Such improvements can provide a moreefficient movement and storage of larger charged ions in the dischargereaction, thereby increasing the net capacity of the battery.

Alternatives to aluminum anodes, such as aluminum metal sulfides andaluminum metal oxides can potentially offer higher operating voltages,associated with the high activation energy of such compounds. Moreover,such alternatives also facilitate the incorporation of a mixed,hybrid-ion technology whereby the capacity contribution is availablefrom more than one metal ion, thereby directly increasing the achievablecapacities and hence, available energy densities. Such alternatives toaluminum anodes are also more stable over a wider range of operatingparameters such as mechanical stresses, high/low operating temperaturesand choice of electrolytes.

Examples of the chemical reactions associated with aluminum metal oxidesis provided below:

Al₂CoO₄→AlCoO₄+Al³⁺+3e  (1)

Where cobalt changes its oxidation state from Co³⁺ to Co⁵⁺ following thedissociation reaction of cobalt aluminate and the release of onealuminum ion.

AlLi₃CoO₂→CoO₂+Al³⁺+3Li⁺+6e  (2).

Cobalt changes its oxidation state from Co²⁺ to Co⁴⁺ following thedissociation reaction and the release of one aluminum ion and threelithium ions.

In embodiments using aluminum compounds comprising alkali metals, thefollowing reactions can be considered.

AlH₄ ¹⁻+Na¹⁺→NaH+AlH₃  (3).

AlF₆ ³⁻+3Li¹⁺→Li₃F₃AlF₃  (4).

Aqueous electrolytes containing a dispersion of alternate aluminum salts(such as sulfates, phosphates and perchlorates) can effect changes inionic mobility and operating voltages.

In addition to aqueous electrolytes, ionic electrolytes offer theability to achieve significantly higher operating voltages(typically, >5 V), thereby increasing the achievable energy density(defined as the product of charge storage and operating voltage). Solidelectrolytes, comprising aluminum and aluminum-metal-based saltsdispersed in polymers such as polyethylene oxides are used in thedisclosed aluminum ion battery chemistry. Solid electrolytes are lowcost alternatives to ionic electrolytes that allow reasonably highoperating voltages along with a marked improvement in terms of ionicmobility.

In certain embodiments, the separator is a porous polypropyleneseparator or a nylon membrane separator that provides an insulatinglayer between the anode and cathode along and provides sufficientporosity for the efficient transport of ions between the two electrodes.In certain embodiments, the separator comprises a porous polymermaterial selected to provide the needed functionality at lesser expense,which can significantly drive down the cost of the technology.

The onset, extent and impact of electrolysis within the batterychemistry has been studied. The current set of operating parameters relyon relatively low voltages at which electrolysis will be absent or atthe most, inconsequential. However, the choice of electrodes andelectrolytes bring in a sufficient degree of thermodynamic non-idealityfactor which can increase the electrolysis voltage to as much as 1.8 V(instead of 1.23 V). Such high operating voltages with aqueouselectrolytes not only enable higher capacities but can also lead to theintroduction of additional reaction mechanisms which were otherwiseunavailable at voltages less than 1.2 V. In terms of hybrid ion batterychemistries, higher operating voltage can also introduce the involvementof multiple metal ions such as lithium ions from the lithium manganeseoxide cathode. Further studies throughout an entire range of operatingparameters to determine the characteristic responses of the batterytechnology.

Cathode Materials. Two cathode materials were studied in workingexamples: acid-treated lithium manganese oxide and graphite-graphiteoxide composite. A suitable cathode material should have sufficientporosity and inter-sheet voids to accommodate the insertion andintercalation of large polyatomic aluminum anions. The cathode materialshould also be partially hydrophilic for wettability with an aqueouselectrolyte. These two cathode materials meet these criteria, but othermaterials, notably graphene and manganese dioxide, could also meet thesecriteria.

Pristine graphite cathodes do not typically provide large inter-sheetvoids and porosity or hydrophilicity. While oxygen plasma treatedgraphite could improve the hydrophilicity of the cathode material, thereis an issue of whether inter-sheet voids would accommodate largepolyatomic aluminum anions.

The spinel structure of lithium manganese oxide provides hydrophilicityas well as porosity and inter-sheet voids suitable for accommodatinglarge polyatomic aluminum anions. Similarly, graphite-graphite oxidecomposites were found to be suitable cathode materials for the proposedaluminum-ion chemistry, owing to the hydrophilicity and largeinter-sheet voids introduced by the oxygen atoms.

One of ordinary skill would recognize that graphene also hassufficiently large inter-sheet voids, owing to the precursor grapheneoxide material, which is subsequently reduced to increase conductivityand form graphene. However, graphene is inherently hydrophobic andtherefore, graphene sheets need to be exposed to oxygen plasma tointroduce oxygen containing species and improve hydrophilicity.Additional alternate cathode materials capable of accommodating thediffusion and storage of large aluminum-based ions, such as manganesedioxide, are being studied to evaluate on the cost, porosity andinter-sheet voids of the potential materials.

Lithium manganese oxide materials can be improved by leaching lithiumatoms from the lithium manganese oxide materials through treatment withacids. When the lithium manganese oxide material is treated with amineral acid such as nitric acid or hydrochloride acid, the lithiumatoms are removed in the form of the corresponding lithium nitrates orlithium chlorides, thereby creating additional voids within the cathodestructure. Suitable acids include aqueous solutions of 10%-90% nitric,hydrochloric, sulfuric, acetic, hydroiodic, or phosphoric acid. In oneembodiment, lithium atoms can be removed by dispersing lithium manganeseoxide in an acidic medium (30%-70% concentrated hydrochloric or nitricacids) and sonicated for 1-6 hours until a stable dispersion isobtained. If nitric acid is used the color of the lithium manganeseoxide changes from black to dark red. In another embodiment, lithiumatoms can be removed by stirring the suspension of dispersing lithiummanganese oxide in an acidic medium at room temperature for about 0.5 toabout 6 hours, typically about two hours. In other embodiments, thesuspensions were stirred for various durations from 0.5 hours to 24hours at different temperatures from room temperature to about 80° C.The resulting suspension is then filtered through a filter membrane withpore size ranging from 0.1-40 μm, preferably with a pore size greaterthan 0.2 μm and is repeatedly washed with deionized water or ethanol toremove trace amounts of residues, such as LiNO₃. Suitable filtermembrane materials, such as nylon, are those that can withstand the acidused in the process. The filtrate is then dried in a vacuum furnace toobtain the resultant powder, Li_(1-x)MnO₂, where x denotes the amount oflithium removed by the acid treatment process. As the acid treatmentapproaches complete removal of lithium, x approaches 1, and the productis substantially MnO₂.

In certain embodiments, commercially available lithium manganese oxideis dispersed in 30% concentrated hydrochloric acid or 67% concentratednitric acid and sonicated for six hours until a stable dispersion isobtained. Formation of a dispersion is indicated by a change in colorfrom black to reddish brown. The resulting suspension is then filteredthrough a Whatman nylon membrane filter with pore size ranging from0.2-0.45 μm and is repeatedly washed to remove trace amounts of theacid. The filtrate is then dried in a vacuum furnace at 110° C. toobtain the resultant powder, Li_(1-x)MnO₂.

Batteries were fabricated that used the aluminum based electrochemistry,including an aluminum anode and an aqueous solution of an aluminum saltas an electrolyte. In certain embodiments, batteries contained analuminum anode, an acid-treated lithium manganese oxide cathode and anaqueous solution of an aluminum salt as an electrolyte in a coin cellconfiguration. In certain embodiments, batteries contained an aluminumanode, a graphite-graphite oxide composite cathode and an aqueoussolution of an aluminum salt as an electrolyte in a coin cellconfiguration. In certain embodiments, batteries contained an aluminumanode, a graphene cathode and an aqueous solution of an aluminum salt asan electrolyte in a coin cell configuration. When a 1 M aqueous solutionof aluminum nitrate was used as the electrolyte, the batteries having analuminum anode and an acid-treated lithium manganese oxide cathodechemistry provided an open circuit voltage of about 1 volt. When a 1 Maqueous solution of aluminum nitrate was used as the electrolyte, thebatteries having an aluminum anode and graphite-graphite oxide compositecathode provided an open circuit voltage between 600 mV and 800 mV. Whena 1 M aqueous solution of aluminum nitrate was used as the electrolyte,the batteries having an aluminum anode and a graphene cathode providedan open circuit voltage between 600 mV and 800 mV. When the cells weredischarged, aluminum-based ions migrated from the anode towards thecathode. Following a discharge, when the cells were charged, aluminumions migrated to the anode.

WORKING EXAMPLES Example 1 Alkali Metal Hydroxide Treatment of theAluminum Anode

Battery-grade pristine aluminum foils were treated to increasehydrophilicity and improve wettability of the anode. While not wishingto be bound by theory, it is believed that a hydrophobic aluminum anodewould prevent efficient ion transport due to high interfacial resistancebetween the aluminum metal and aqueous electrolyte interface, leading toa significant drop in performance. The hydrophilicity of the surface ofthe aluminum metal was increased by treatment with a lithium hydroxideaqueous solution. Lithium is known to have a strong affinity foraluminum, forming a range of lithium-aluminum based alloys,characterized by the formation of a greyish-white texture on thealuminum surface. The resulting aluminum anode is found to besignificantly more hydrophilic than untreated, pristine aluminum.Aqueous solutions of other alkali metal hydroxides, such as sodiumhydroxide and potassium hydroxide, are also suitable for use in thistreatment.

FIG. 1A is a photograph of a piece of aluminum foil treated with a dropof 1 M aqueous solution of lithium hydroxide; FIG. 1B is a photograph ofthe piece of the treated aluminum foil of FIG. 1A showing a change inthe appearance of the drop of the aqueous solution of lithium hydroxide.After a short time the aqueous solution of lithium hydroxide was wipedaway and the surface of the aluminum foil was allowed to air dry at roomtemperature (25° C.). Reaction times of 5-10 seconds to 1 hour have beentested, but the reaction appears to be complete in 5-10 seconds. FIG. 1Cis a photograph of the piece of treated aluminum foil of FIG. 1B showingthe greyish-white appearance of the aluminum foil following the dryingof the lithium hydroxide solution. FIG. 1D is a photograph of the pieceof treated aluminum foil of FIG. 1C showing the effect of placing a dropof deionized water on the treated aluminum foil, indicating an increasein hydrophilicity of the treated aluminum foil. FIG. 1E is a photographof a drop of deionized water on untreated aluminum foil for comparisonto FIG. 1D. In certain embodiments, an aqueous solution of about 0.01Mto about 5.5M lithium hydroxide can be used.

Other means of increasing the hydrophilicity of an aluminum metalsurface, such as nitrogen and oxygen plasma and treatment usingacid-based treatments, primarily rely on the introduction of hydrophilicnative aluminum oxide layers on the metal surface. Acid-based treatmentsinvolve complicated chemistry, increasing manufacturing costs, and havesignificant environmental impacts associated with use and disposal.Plasma treatment, on the other hand, is an expensive high voltageprocess, and is unsuitable for large-scale manufacturing.

Example 2

Aluminum Anode vs. Acid-Treated Lithium Manganese Oxide Cathode Batteryand Aluminum Anode vs. Graphite-Graphic Oxide Cathode Battery

Batteries were assembled using an aluminum anode, an acid-treatedlithium manganese oxide cathode and an aqueous solution of aluminumnitrate as the electrolyte. Studies were conducted using the standard2032 coin cell form factor, illustrated in FIG. 2. FIG. 2 is a schematicdiagram of an exploded view of a test battery 100 in a coin cell format,showing the positive case 110, a spring 120, a first spacer 130, acathode 140, a separator 150, an anode 160, a second spacer 170 and thenegative case 180. Prior to assembly of the test battery 100, aliquotsof the electrolyte are placed between the separator 150 and the anode160, as well between the separator 150 and the cathode 140. Preferably,the first spacer 130, the cathode 140, the separator 150, the anode 160,and the second spacer 170 are immersed in and equilibrated with theelectrolyte prior to assembly of the test battery.

A battery grade aluminum foil is used as the anode 160. Battery gradefoils are generally >99% pure. The thickness of any battery-grade foilshould be limited, since the thickness directly impacts the volumetricenergy density at the system level, defined as: (Net Available EnergyDensity in Watt-hours/Total volume of the electrode, including thecurrent collector). Thicker current collectors also reduce the maximumnumber of electrodes that can be stacked in a battery pack/module.Ideally, battery-grade current collectors vary between 8-30 μm inthickness. Mechanical robustness is also necessary to prevent any wearand tear during the electrode coating or cell/battery assembly process.The tensile strength of commercial battery-grade foils vary between100-500 N/mm. Suitable battery grade aluminum foil and other materialsmay be obtained from MTI Corporation, Richmond, Calif. and TargrayTechnology International Inc., Laguna Niguel, Calif. Battery gradelithium manganese oxide cathode and graphite may be obtained from MTICorporation, Richmond, Calif. and Sigma Aldrich, St. Louis, Mo. Grapheneand graphite oxide may be purchased from Sigma Aldrich, St. Louis, Mo.,Graphene Supermarket, Calverton, N.Y., and ACS Material, Medford, Mass.

Aluminum foil anodes were treated as described in Example 1 to improvetheir hydrophilic properties.

The charge/discharge steps were carried out in the voltage window of0-2V. As the cell was discharged, hydroxyaluminate (Al(OH)₄ ⁻) ions wereformed according to chemical reaction (5):

Al³⁺+4OH⁻→Al(OH)₄ ⁻  (5)

The hydroxyaluminate ions migrate towards the cathode, passing throughthe porous membrane separator.

At the cathode, the hydroxyaluminate ions diffuse through the pores andinter-sheet voids of the cathode material and are oxidized to giveAl(OH)₃ (aluminum hydroxide). The presence of aluminum hydroxide on thealuminum foil anode of a completely (100%) discharged test cell has beenconfirmed using x-ray photoelectron spectroscopy (XPS), as shown in FIG.3. The XPS profile shows one major Al 2p transition at 74.3 eV,indicating the presence of gibbsite, Al(OH)₃ on the anode. Thetransition at 74.3 eV has been reported to be characteristic of gibbsiteby Kloprogge et al. Kloprogge, J. T., et al., XPS study of the majorminerals in bauxite: gibbsite, bayerite and (pseudo-) boehmite, Journalof Colloid and Interface Science, 2006, 296(2), 572-576. In the reversecharging process, aluminum hydroxide is reduced at the cathode and thealuminum ions migrate back to the anode.

The discharge and charge reactions at the cathode, chemical reactions(6) and (7), respectively, are provided below:

Discharge, Al(OH)₄ ⁻→Al(OH)₃+OH⁻+3e  (6);

Charge, Al(OH)₃+3e ⁻→Al³⁺+3OH⁻  (7)

During the charging process, an additional contribution may be observedat the acid-treated lithium manganese oxide cathode through thedissociation reaction of lithium manganese oxide according to reaction(8), below:

LiMnO₂ +e ⁻→Li⁺+MnO₂  (8).

The lithium ions would then flow towards the aluminum anode and possiblyintercalate with aluminum, owing to the high affinity between lithiumand aluminum, forming a hybrid-ion battery chemistry. This hypothesiscould relate to an observed increase in capacity (about 40%) compared tothe capacities obtained with carbon-based cathodes devoid of any lithiumcomponent. However, XPS examination of the aluminum anode in a test cellhaving a lithium manganese oxide cathode did not show any significantsigns of lithium-based alloys at the anode site at a fully chargedstate. However, this result can be attributed to the fact that thedissociation reaction of lithium manganese oxide occurs at significantlyhigher voltages (>3V) and in the given voltage window, the concentrationof lithium ions is negligible compared to the presence of aluminum-basedalloys. In addition, XPS is a surface-based analytic technique and theexcellent lithium ion diffusion in aluminum might have caused thelithium ions to have diffused within the bulk aluminum anode and wouldtherefore be absent from the surface.

Operating Parameters and Performance Metrics. The aluminum-ion cellswere cycled between safe voltage cut-off limits of 0 V (discharge) and 2V (charge). However, the average operating voltage was about 1.1 V fordischarge and 1.2 V for charge for the cells having an aluminum anodeand an acid-treated lithium manganese oxide cathode (FIG. 4A) and 0.4 Vfor discharge and 0.9 V for charge for the cells having an aluminumanode and a graphite-graphite oxide cathode (FIG. 4B), within the safevoltage cut-off limits.

FIG. 4A shows the voltage profile that was produced by applying currentat a current density of 0.1 mA/cm² to a test battery having an anodecomprising an aluminum foil treated with LiOH as described in Example 1,a cathode comprising acid-treated lithium manganese oxide, a 25 μm thickpolypropylene separator with an average pore size of 0.067 μm, and a 0.5M aqueous aluminum nitrate electrolyte. FIG. 4B shows the voltageprofile that was produced by applying current at a current density of0.1 mA/cm² to a test battery having an anode comprising an aluminum foiltreated with LiOH as described in Example 1, a cathode comprisinggraphite-graphite oxide, a 25 μm thick polypropylene separator with anaverage pore size of 0.067 μm, and a 0.5 M aqueous aluminum nitrateelectrolyte. The observed average operating voltage is significantlyhigher with the use of acid-treated lithium manganese oxide cathodes,possibly owing to the higher activation energy for diffusion andintercalation of ions. Carbon is known to possess a sufficiently lowactivation energy for diffusion and intercalation of metal ions (theintercalation voltage of lithium ions in carbon against a lithium metaloccurs at about 100 mV).

Moreover, the voltage profile demonstrates a characteristic voltageplateau (FIG. 4A), unlike the voltage profiles observed in sodium ionbatteries, enabling critical advantages such as incorporation of simplerbattery management systems and installation of fewer cells in seriesowing to the high operating voltages, all of which can significantlydrive down the cost of the technology. The charge/discharge rates werelimited between C/1 and C/12 (a rate of C/n implies charge or dischargein n hours). While cycle life testing is currently underway, both thegraphite-based and acid-treated lithium manganese oxide-basedconfigurations have so far demonstrated impressive cycle life,delivering close to about 100% coulombic efficiency (defined as theratio of charge to discharge capacities and is indicative ofirreversibility and side-reactions in a battery chemistry). FIG. 5Ashows the battery charge capacity, open triangles, and dischargecapacity, open circles, as a function of the cycle index of a batteryhaving an anode comprising an aluminum foil treated with LiOH asdescribed in Example 1, a 25 μm thick polypropylene separator with anaverage pore size of 0.067 μm, and a 0.5 M aqueous aluminum nitrateelectrolyte and a graphite-graphite oxide composite cathode. Thecoulombic efficiency was estimated to be close to 100% over 30charge/discharge cycles, indicating efficient and reversible charge anddischarge kinetics.

FIG. 5B shows the battery charge capacity as a function of cycle indexof a battery having an anode comprising an aluminum foil treated withLiOH as described in Example 1, a 25 μm thick polypropylene separatorwith an average pore size of 0.067 μm, and a 0.5 M aqueous aluminumnitrate electrolyte and an acid-treated Li_(1-x)MnO₂ cathode. Thereduction in capacity after over 800 charge/discharge cycles is about 3%of the original capacity.

Understandably, the acid-treated lithium manganese oxide-aluminumchemistry with an aqueous electrolyte provides higher energy density(about 100-150 Wh/kg) and volumetric energy density (about 30-60 Wh/L)than the graphite-graphite oxide-aluminum chemistry (about 50-75 Wh/kgand about 20-30 Wh/L), normalized by the mass and volume of cathode,owing to the higher electrochemical affinity towards aluminum observedin acid-treated lithium manganese oxide. However, graphite-graphiteoxide cathodes are generally cheaper than lithium manganese oxide.Further modifications to the cathode chemistry may significantly boostthe performance metrics of graphite-graphite oxide cathodes.

Sequential cyclic voltammetry tests were carried out to measure the iondiffusion coefficient in batteries having an anode comprising analuminum foil treated with LiOH as described in Example 1, a 25 μm thickpolypropylene separator with an average pore size of 0.067 μm, and a 0.5M aqueous aluminum nitrate electrolyte and an acid-treated lithiummanganese oxide cathode (FIG. 6A) or batteries having an anodecomprising an aluminum foil treated with LiOH as described in Example 1,a 25 μm thick polypropylene separator with an average pore size of 0.067μm, and a 0.5 M aqueous aluminum nitrate electrolyte andgraphite-graphite oxide composite cathode (FIG. 6B). The test batteriesin the coin cell format were cycled at various voltage sweep ratesbetween 10 mV/sec and 50 mV/sec within a voltage range of 0 V and 1.5 V.

The diffusion coefficient was calculated using Fick's law followingequation (I):

$\begin{matrix}{\frac{\partial C}{\partial t} = {{D\frac{\partial^{2}C}{\partial r^{2}}} + {\frac{2D}{r} \cdot {\frac{\partial C}{\partial r}.}}}} & (I)\end{matrix}$

The response of current can then be obtained as:

$\begin{matrix}{{i = {\frac{nFADC}{R_{0}} + \frac{nFAD^{1/2}C}{\pi^{1/2}t^{1/2}}}}.} & ({II})\end{matrix}$

Where n is the number of electrons exchanged, F is Faraday's constant, Ais the area of the electrode, D is the diffusion coefficient, C is themolar concentration, t is time for diffusion and R₀ is the radius of thecathode particles.

The current response can be simplified and re-written as:

$\begin{matrix}{{i = {{kt^{1/2}} + b}}{where}} & ({III}) \\{{b = \frac{nFADC}{R_{0}}}{{k = \frac{{nFAD}^{1/2}C}{\pi^{1/2}}},}} & \left( {{IV},V} \right) \\{{or},{D = \frac{b^{2}R_{0}^{2}}{\pi \; k^{2}}}} & ({VI})\end{matrix}$

Subsequently, the diffusion coefficient of hydroxyaluminate ions inacid-treated lithium manganese oxide and graphite-graphite oxidecomposite cathodes was calculated to be 1.14×10⁻⁷ cm²/sec and 3.54×10⁻⁸cm²/sec respectively, well within the acceptable range of ion diffusioncoefficients in metal-ion batteries.

In addition, electrochemical impedance spectroscopy (EIS) was carriedout to analyze the internal resistances of batteries having an anodecomprising an aluminum foil treated with LiOH as described in Example 1,a 25 μm thick polypropylene separator with an average pore size of 0.067μm, and a 0.5 M aqueous aluminum nitrate electrolyte and an acid-treatedlithium manganese oxide cathode (FIG. 7A) and batteries having an anodecomprising an aluminum foil treated with LiOH as described in Example 1,a 25 μm thick polypropylene separator with an average pore size of 0.067μm, and a 0.5 M aqueous aluminum nitrate electrolyte and agraphite-graphite oxide cathode (FIG. 7B). Insets show the Randlesequivalent circuit used to fit the spectra.

Since the operating voltages are close to the electrolysis voltage ofthe aqueous electrolyte system, analyzing EIS is critical to understandpotential safety threats associated with gas evolution inside the cell.It is understood that the evolution and presence of gas pockets (H₂ andO₂) within the electrolyte will directly increase the electrolyticresistance while the evolution of these gases at theelectrode-electrolyte interface will increase the interfacialresistance.

The EIS profile was fitted with a Randles equivalent circuit model andthe electrolytic resistance, interfacial resistance and charge transferresistances, summarized in Table 2, below, were estimated based on thefit. The electrolytic resistances were estimated to be between 2-4Ω,significantly lower than the typical electrolytic resistances of 10-20Ω,consistent with the absence of any gas pockets within the electrolyte.The interfacial resistance was estimated to be 11Ω at the acid-treatedlithium manganese oxide-electrolyte interface and 20Ω at thegraphite-graphite oxide-electrolyte interface, again consistent with theabsence of any insulating gas pockets. The charge transfer resistance ofacid-treated lithium manganese oxide was estimated to be about 100Ωwhile that of graphite-graphite oxide composite was estimated to beslightly higher at about 131Ω, possibly attributed to the presence ofoxygen-containing functional groups. Charge transfer resistance isindicative of the electron conductivity of the active electrode materialand is independent of the formation of gas pockets. One of ordinaryskill would recognize that a charge-transfer resistance of 100-150Ω isgenerally considered to be suitable for battery storage applications.

TABLE 2 EIS Profile Results Acid-Treated Lithium Graphite-Graphite OxideParameter Manganese Oxide Cathode Cathode Rel 2-4 Ω 2-4 Ω Rint  11 Ω  20Ω Ret about 100 Ω about 131 Ω

In order to assess form factor scalability of the aluminum ion batterychemistry, pouch cell and prismatic cells were also assembled andtested. A schematic depiction of the prismatic cell assembly is providedin FIG. 8A. The prismatic assembly 80 comprised a metallic or a polymerbase plate 82 an insulating polymer gasket 84 and a top plate resemblingthe structure of the base plate, not shown for clarity. The componentshad threaded through-holes along its edges. For metallic base and topplates, nylon screws were used to seal the assembly while simultaneouslypreventing a shorting between the two plates while for polymer base andtop plates, both nylon and metallic screws sufficed. A prismatic cellwas assembled with an electrode area of 2 cm×5 cm and rated at acapacity of 1 mAh. Standard polypropylene separators were used in theassembly. The cell comprised a single interface, although multipleinterfaces can also be incorporated with the setup.

FIG. 8B illustrates the discharge voltage profile of a prismatic cellrated at 1 mAh. The cell had an anode comprising an aluminum foiltreated with LiOH as described in Example 1, a cathode comprisingacid-treated lithium manganese oxide, a 25 μm thick polypropyleneseparator with an average pore size of 0.067 μm, a 0.5 M aqueousaluminum nitrate electrolyte and were tested at 10 μA/cm².

Pouch cells were assembled by introducing the anode-separator-cathodeinterface in the cell assembly section of an aluminum laminate pouchcell packaging case. This was followed by connecting the electrodes toan aluminum current collector tab through mechanical contacts orultrasonic welding. Next, three edges of the pouch cell were sealedusing a heat sealer set between about 20-50 psi and 150-180° C. Asection of the pouch cell was retained at one of the edges that acted asthe gas trap. The purpose of the gas trap is to contain the gasevolution during the formation cycle, after which the gas trap sectioncan be cut and the edge resealed for subsequent cycling. Prior tosealing the fourth and final edge of the pouch cell, theelectrodes-separator assembly was wetted with the electrolyte. Thefourth edge is sealed in a vacuum sealing furnace with the vacuum set atabout −90 psi. The pressure and temperature parameters are unchanged andare set between 20-50 psi and 150-180° C. respectively. A schematicdiagram of a pouch cell 90 is provided in FIG. 9A, showing a cellassembly section 92, a gas trap 94 and current collector terminals 96. Apouch cell assembled in this fashion involved electrodes between 1cm×0.8 cm and 1 cm×1 cm, rated between 0.06-0.08 mAh.

FIG. 9B illustrates the discharge profile of a pouch cell comprising 0.8cm×1 cm electrodes and hydrophilic polypropylene separators. The cellhad an anode comprising an aluminum foil treated with LiOH as describedin Example 1, a cathode comprising acid-treated lithium manganese oxide,a 25 μm thick polypropylene separator with an average pore size of 0.067μm, a 0.5 M aqueous aluminum nitrate electrolyte and were tested atabout 25 μA/cm². Like the prismatic cell format, a pouch cell assembledin this fashion can also incorporate scalability and multiple interfacescan be packed in series/parallel configurations to achieve apre-determined capacity and voltage rating.

The need to find an alternate energy storage system to meet theever-increasing demands from various sectors such as consumerelectronics, military, automotive and grid storage have continued torise exponentially in the last decade. While lithium ion batteries areubiquitous today in consumer electronics, its limitations with respectto high costs and potentially hazardous safety threats have limited itsentry in emerging fields such as grid storage, and automotiveapplications. Sodium ion batteries on the other hand may offer amarginal reduction in the cost at the system level but are significantlylimited in performance metrics in terms of available capacities andenergy density, voltage window and the choice of electrolytes. Alternateupcoming technologies such as flow batteries and liquid metal batteriesare still in early stages of development. Moreover, such technologiespose additional challenges in terms of cost of implementation andsafety. For instance, the availability of unlimited capacity in vanadiumredox flow batteries rely on large storage tanks and industrial pumpsthat add to the cost and maintenance of the battery system. Liquid metalbatteries operate at very high temperatures and incorporate the use oftoxic materials such as antimony and lead as well as flammable lithiummetal, thereby posing serious safety concerns.

Also disclosed are systems and methods of using the disclosedrechargeable battery. FIG. 8 is a block diagram of an embodiment of asystem 800 that incorporates the battery 810 of the present disclosure,showing a controller 820 that is operatively connected to battery 810, asource of electrical power 830, a local electrical load 840 and anelectrical power distribution grid 850. In certain embodiments, thesource of electrical power 830 is based on a renewable energy source,that is, a wind turbine or a solar panel. The controller 820 isoperatively connected to the source of electrical power 830 and to thebattery 810 of the present disclosure to mediate the charging of thebattery 810. The controller 820 is operatively connected to the sourceof one or more local electrical loads 840 and to the battery 810 of thepresent disclosure to mediate the discharging of the battery 810. Thelocal electrical loads 840 can include devices requiring DC electricalsupply or AC electrical supply, including, without limitation, cellphone or computer battery chargers, computers, home appliances, waterpumps, and refrigeration equipment. In certain embodiments, thecontroller 820 is operatively connected to a power distribution grid 850to permit selling excess electrical power to the power distribution grid850.

Example 3

Electrolyte Additives

In some examples, the electrolyte, 0.5 M Al(NO₃)₃, was mixed with 10-50vol. % 2 M LiOH to obtain a composite electrolyte. While not wishing tobe bound by theory, it is believed that addition of LiOH to theelectrolyte comprising aluminum ions increases the concentration of OH⁻¹in the electrolyte, and therefore prevents loss of active ions duringtransportation through undesired side reactions. Owing to the reactivityof aluminum in water, it is not uncommon to observe oxidation of Al(OH)₄¹⁻ ions prior to reaction at the cathode site.

The oxidation reaction can be summarized as:

Al(OH)₄ ¹⁻→Al³⁺+4OH⁻¹  (9).

Depending on the concentration of OH⁻¹ ions in the electrolyte, therecan be a reduction of active hydroxyaluminate ions reaching the cathode,through a dissociation reaction that subsequently leads to the oxidationof hydroxyaluminate and re-formation of multivalent aluminum ions.Increasing the concentration of OH⁻¹ ions through incorporation of 2 MLiOH in the electrolyte would shift the equilibration of the reaction tofavor Al(OH)₄ ¹⁻ ions over the dissociation to Al³⁺+4OH⁻¹.

FIG. 11 compares the discharge and charging properties of two batteriesdiffering in electrolyte composition: one battery having a 0.5 MAl(NO₃)₃ (aq) electrolyte (curve 1) and another battery having a 0.5 MAl(NO₃)₃ and 2 M LiOH (aq) electrolyte (curve 2). Each battery wasassembled in a 2032 coin cell format and had an anode comprising analuminum foil treated with LiOH as described in Example 1, a cathodecomprising acid-treated lithium manganese oxide, a 25 μm thickpolypropylene separator with an average pore size of 0.067 μm, and wastested at current densities of 10 μA/cm². In certain embodiments, theelectrolyte is an aqueous solution of aluminum nitrate and lithiumhydroxide in a molar ratio of about 1:1 to about 1:10.

The results illustrated in FIG. 11 suggest that incorporation of acomposite electrolyte can result in a reduced over-potential.Incorporation of LiOH in the electrolyte reduces the over-potential bypreventing loss of active Al-ion species during transportation in theelectrolyte. The over-potential relates to the discharge voltagehysteresis caused by the cell composition. The voltage hysteresis duringdischarge is defined as the change from the expected operating voltageattributed to internal cell resistances and is conveyed through theequation ΔV=IR (where, ΔV is the change in voltage, I is the current andR is the internal resistance). The new operating voltage, Vop′, is thendefined as Vop′=Vop−ΔV. In FIG. 11, curve 1, in which the electrolyte is0.5 M (aq) Al(NO₃)₃ with no LiOH additive, the higher internalresistances cause an increase in the ΔV value (which is known as theover-potential), causing it to discharge at a lower voltage. Incontrast, the battery having an electrolyte that is 0.5 M Al(NO₃)₃ and 2M LiOH aqueous solution (curve 2) has a reduced internal cellresistance, therefore causing the ΔV value to be less than that of thebattery of curve 1, resulting in a higher discharge potential (whichimplies higher energy density, since energy density=capacity x operatingvoltage)

While the example describes a LiOH—Al(NO₃)₃ composite electrolyte, otherhydroxide-containing compounds may also be introduced into theelectrolyte to achieve similar effects. Some alternate electrolyticadditives include hydroxides of sodium, potassium, ammonium, calcium andmagnesium. Since the alkali metal ion (Li+, Na+ or K+ for example) ispresent only in the electrolyte and has a polarity opposite to Al(OH)₄¹⁻ions, it will not contribute to discharge capacities.

Example 4 Separators

The aluminum ion chemistry disclosed herein involves the transport oflarge Al(OH)₄ ¹⁻ ions. Therefore, the pore size of the separator candictate the ion transportation kinetics. Standard batteries such aslithium ion batteries generally use a polypropylene separator with poresizes less than 0.1 μm, which is sufficient to permit the flow of therelatively smaller lithium ions. However, as the size of the ionsincrease, as is the case with Al(OH)₄ ¹⁻, small pore sizes hinder theefficient flow of ions, resulting in an increased internal cellresistance and lower charging rate and discharging rate. Therefore, inan effort to reduce accumulation of charge and resistance build-up atthe separator surface, batteries having separators with larger porediameters were studied.

The separators that were tested included polypropylene separators(standard, 0.067 μm pore size; Celgard LLC, Charlotte, N.C.), mixedcellulose ester separators (0.2 μm pore size, Whatman), nylon separators(0.45 μm, 0.8 μm, 1.2 μm pore sizes, Whatman), and glass microfiberseparators (1 μm pore size, Whatman). In general, larger pore sizes,including 0.8 μm and 1.2 μm, permitted faster ion-transfer kinetics andbetter rate capabilities compared to the smaller pore sizes. However asthe pore sizes were further increased, it appeared that there was anincrease in shorting of the anode and cathode that was noticeable at apore diameter of 2.7 μm.

FIG. 12 illustrates the effect of separator pore size on the averagedischarge potential produced at a given current density, where pentagons(1) represent measurements made on a battery having a polypropyleneseparator with 0.067 μm pores, a triangle (2) represents measurementsmade on a battery having a mixed cellulose ester separator with 0.20 μmpores, a circle (3) represents measurements made on a battery having anylon separator with 0.45 μm pores, squares (4) represent measurementsmade on a battery having a nylon separator with 0.80 μm pores, anddiamonds (5) represent measurements made on a battery having a glassmicrofiber separator with 1.0 μm pores. Each battery was assembled in a2032 coin cell format and had an anode comprising an aluminum foiltreated with LiOH as described in Example 1, a cathode comprisingacid-treated lithium manganese oxide, and the electrolyte was an 0.5 Maqueous aluminum nitrate solution. Polypropylene separators (pentagons)were tested at 10 μA/cm² and 20 μA/cm²; mixed cellulose ester separators(triangle) and nylon separators (circle) were tested at 20 μA/cm²; nylonseparators (squares) were tested at 20 μA/cm², 40 μA/cm² and 50 μA/cm²;and glass microfiber separators (diamonds) were tested at 20 μA/cm² and40 μA/cm².

The pore size of the separators can also influence suitability of abattery for an intended application. Understandably, separators withlarger pore sizes are also thicker than those with smaller pore sizes.Therefore, while the range of optimum pore sizes is rather large, aspecific choice can be made based on the intended application. Forexample, smaller pore sizes (example, polypropylene, 0.067 μm porediameter and 25 μm thick) can optimize energy density (volumetric andgravimetric) while larger pore sizes (example, glass microfiber, 1 μmpore diameter and 500 μm thick) can optimize rate capability and hence,improve the power density of such batteries.

FIG. 13 illustrates the discharge of a battery having a polypropyleneseparator with 0.067 μm pores at a current density of 10 pA/cm². Thebattery was assembled in a 2032 coin cell format and had an anodecomprising an aluminum foil treated with LiOH as described in Example 1,a cathode comprising acid-treated lithium manganese oxide, and theelectrolyte was an 0.5 M aqueous aluminum nitrate solution.

FIG. 14 illustrates the discharge of a battery having a nylon separatorwith 0.80 μm pores at a current densities of 20 pA/cm² (curve 1), 40pA/cm² (curve 2), and 40 pA/cm² (curve 3). The battery was assembled ina 2032 coin cell format and had an anode comprising an aluminum foiltreated with LiOH as described in Example 1, a cathode comprisingacid-treated lithium manganese oxide, and the electrolyte was an 0.5 Maqueous aluminum nitrate solution.

FIG. 15 illustrates the discharge of a battery having a glass microfiberseparator with 1.0 μm pores at a current densities of 20 pA/cm²(curve 1) and 40 pA/cm² (curve 2). The battery was assembled in a 2032coin cell format and had an anode comprising an aluminum foil treatedwith LiOH as described in Example 1, a cathode comprising acid-treatedlithium manganese oxide, and the electrolyte was an 0.5 M aqueousaluminum nitrate solution.

In addition, as a result of lowered internal resistance, separators withlarger pore sizes enabled a higher voltage of operation, even atsignificantly higher current densities. For example, at a currentdensity of about 20 pA/cm², a polypropylene separator (0.067 μm poresize), a nylon separator (0.8 μm pore size) and a glass microfiberseparator (1 μm pore size) displayed an average discharge potential of1.01 V, 1.17 V and 1.18 V respectively. See FIG. 12. Table 3, below,summarizes the average charge and discharge potential and the typicalcharge and discharge voltage hysteresis values for batteries constructedwith various separators, compared against baseline (standard 0.067 μmpore size polypropylene separator tested at a current density of about10 pA/cm²).

TABLE 3 Charge & Discharge Characteristics For Specific CurrentDensities And Separators Current Average Discharge Average ChargeDensity Discharge Hysteresis Charge Hysteresis Separator (μA/cm²)Voltage (V) (ΔV) Voltage (V) (ΔV) Polypropylene (0.067 μm) 10 1.11 301.20 40 Mixed cellulose ester (0.2 μm) 20 1.11 40 1.26 50 Nylon (0.45μm) 20 1.12 60 1.33 100 Nylon (0.8 μm) 20 1.17 53 1.34 72 40 1.03 991.40 221 Glass Microfiber (1 μm) 20 1.18 47 1.38 64 40 1.09 72 1.39 109

Table 3 shows that batteries having nylon or glass microfiber separatorsproduced higher voltages (1.17 V-1.18 V) compared to standardpolypropylene separators (1.11 V) even at twice the current density. Infurther studies, the discharge and charge hysteresis voltages (voltagedifference between end-of-charge and start-of-discharge andend-of-discharge and start-of-charge respectively) were found to bewithin sufficiently acceptable values even at four-fold higher currentdensities (data not shown).

Other suitable separators also include, but are not limited to,polyvinylidene fluoride, polytetrafluoroethylene, cellulose acetate,nitrocellulose, polysulfone, polyether sulfone, polyacrylonitrile,polyamide, polyimide, polyethylene, and polyvinylchloride. In addition,anion exchange membranes and proton exchange membranes such as NAFION®may be used as the separator. Ceramic separators including, but notlimited to, alumina, zirconia oxides and silicon oxides can also beused. As identified through the tests, the separators can have a poresize ranging between 0.067 μm and 1.2 However, separators with lower orhigher porosities and thicknesses can also be used for specificapplications.

Example 5 Solid Polymer Electrolytes

As an alternative to the liquid aqueous electrolyte, a solid polymerelectrolyte (SPE) incorporating at least one aluminum salt with orwithout one or more sources of hydroxides can be used in certainembodiments. Not only does the use of SPEs allow higher operatingvoltages, it enables a stable reaction dynamic over a wide range ofoperating conditions (temperature, humidity, mechanical stresses, etc.).While the aluminum salt ensures efficient flow of aluminum ions throughthe electrolyte, added hydroxides contribute OH⁻to enable the formationof Al(OH)₄ ¹ ions during the transportation of ions. Aluminum saltsinclude but are not limited to Al(NO₃)₃, Al₂(SO₄)₃ and AlCl₃ andcombinations and variations thereof. Hydroxides include but are notlimited to Al(OH)₃, LiOH, NaOH, KOH, Ca(OH)₃, Mg(OH)₂ and NH₄OH andmixtures thereof. In general, the polymer is selected from the groupconsisting of polytetrafluoroethylene, acetonitrile butadiene styrene,styrene butadiene rubber, ethyl vinyl acetate, poly(vinylidenefluoride-co-hexafluoropropylene), polymethyl methacrylate, and mixturesthereof.

Aluminum Salt-Based SPEs: In a certain embodiment, a cross-linkingpolymer such as poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP) or polymethyl methacrylate (PMMA) is mixed with aluminumnitrate in a ratio ranging between 1:1 and 1:10. The mixture is thendissolved in a solvent such as N-methyl pyrrolidone or dimethylsulfoxide and heated at temperatures ranging from 50-200° C. underconstant stirring for 2 hours in order to initiate the polymerizationreaction. The solution is observed to turn viscous, following which itis transferred to a vacuum furnace chamber where it is further heated atthe above temperature range for 2-24 hours in order to remove thesolvent and obtain the resultant solid polymer electrolyte. The producedsolid polymer electrolyte comprises the aluminum salt and thecross-linking polymer in the weight ratio that was selected, typicallythe cross-linking polymer at 9-50 wt % and the aluminum salt at 50-91 wt%).

In another embodiment, the cross-linking polymer may be mixed withaluminum halides (such as AlCl₃, AlBr₃, AlI₃) and1-ethyl-3-methylimidazolium chloride (EMIMCl, Sigma-Aldrich),1-ethyl-3-methylimidazolium bromide (EMIMBr, Sigma-Aldrich), or1-ethyl-3-methylimidazolium iodide (EMIMI, Sigma-Aldrich), where theratio of the aluminum halide to the 1-ethyl-3-methylimidazolium halideranges from 1:1 to 5:1 (weight:weight). The combined aluminum halide and1-ethyl methylimidazolium halide is then mixed with the cross linkingpolymer such as PVDF-HFP or PMMA in a ratio of 1:1 to 10:1(weight:weight). Typically, 0.1 to 1 g of the mixture per mL of solventis combined with a solvent such as solvent can be N-methyl pyrrolidoneor DMSO. The mixing and heating steps are similar to the processdescribed above. The resultant solid polymer electrolyte will containaluminum salt, 1-ethyl methylimidazolium halide and the cross-linkingpolymer in the weight ratio that was chosen, typically the cross-linkingpolymer at 9-50 wt % and the aluminum salt at 50-91 wt %.

The mixture dissolved in the solvent is heated between about 50° C. and200° C. for 2-24 hours under constant stirring. In one example, themixture was heated at 90° C. continuously for 2 hours under constantstirring. This step initiates the polymerization reaction. At the end ofthis step, the solution turns viscous indicating successful completionof the polymerization reaction. The mixture is then poured into a flatglass petri dish or other suitable container and transferred to a vacuumfurnace where it is heated between 50° C. and 200° C. overnight or foras long as necessary to completely remove the solvent. At the end ofthis step, a free-standing SPE is obtained that can be released from theglass surface either mechanically (peeling off) or through theapplication of ethanol. A photograph of a SPE is shown in FIG. 16, whichshows the cylindrical, free-standing, translucent solid polymerelectrolyte that is about 1 mm thick and about 3 cm in diameter.

Introduction of Hydroxides in SPEs: The addition of hydroxides to theelectrolyte. described in Example 3, above, can be achieved byintroducing a suitable hydroxide in the mixture in addition to thecross-linking polymer and aluminum salt. In one embodiment, 100 mglithium hydroxide and 900 mg aluminum nitrate were added to about 5 mLdeionized water which resulted in the formation of aluminum hydroxide bythe following reaction:

3LiOH+Al(NO₃)₃→Al(OH)₃+3LiNO₃  (10).

In a separate container, poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP, Sigma-Aldrich) was dissolvedin acetone, at a concentration of 500 mg of the polymer in 5 mL acetone,through bath sonication for up to 6 hours, while the bath itself wasmaintained at a temperature of 60° C. The volume of acetone wasmaintained at 5 mL through subsequent addition of the solvent as andwhen required. Upon dissolution of PVDF-HFP in acetone, 5 mL of thesolution was added to 5 mL of the aqueous electrolyte solutioncomprising the reaction products of lithium hydroxide and aluminumnitrate dispersed in DI water. The addition of the PVDF-HFP solution tothe aqueous electrolyte solution initiated a polymerization reactionwhich resulted in the formation of a free-standing solid polymerelectrolyte as shown in the inset of FIG. 17.

In a particular embodiment, a solid polymer electrolyte prepared by themethod described in the above paragraph was tested in a 2032 coin cellcomprising of a cathode comprising manganese oxide treated by acid-baseddelithiation followed by lithium hydroxide etching of the cathode, andan anode comprising aluminum foil treated as described in Example 1. Noseparators or liquid electrolytes were used and the solid polymerelectrolyte was sandwiched between the anode and cathode.

FIG. 17 illustrates the voltage profile of a battery having asolid-polymer electrolyte, showing a short duration of discharge at 50μA/cm², followed by discharging at 20 μA/cm² and charging at a currentdensity of 20 μA/cm², with an inset of a photograph of solid polymerelectrolytes, indicated by arrows.

Example 6 Charge and Discharge Cycles

An embodiment of a system useful for changing and discharging thedisclosed aluminum ion batteries is illustrated in FIG. 10. In additionto standard galvanostatic (constant current) charge cycles,potentiostatic or a combination of potentiostatic and galvanostaticcharge cycles were shown to have an impact on the performance,specifically in terms of faster reaction kinetics (rate capability). Therange of voltages for potentiostatic charge was identified to liebetween 1.5 V and 2 V, while the optimum value was identified to beabout 1.8 V. At voltages greater than 2 V significant electrolysis wasobserved, confirmed by a rapid rise in currents.

The observed increase in rate capability could be attributed to thepresence of a stronger electromotive force to aid in the transport ofaluminum-based ions from the cathode back to the anode, which wouldcause few or no aluminum ions to be lost through side reactions in theelectrolyte and thereby a steady electric field is maintained to guidethe direction of flow of ions. In addition to galvanostatic,potentiostatic and galvanostatic-potentiostatic charge cycles, aconstant voltage sweep rate can be applied to charge the cell in certainembodiments. In certain embodiments, the dV/dt value of the constantvoltage sweep rate is from 0.01 mV/second to 100 mV/second.Galvanostatic charge and constant voltage sweep rate charge can both beapplied in conjunction with a final constant voltage charge to ensurecompletion of the charge cycle. Typically, in certain embodiments, thefinal constant voltage charge is maintained to achieve trickle chargeuntil the current drops below a pre-determined value ranging from 1% to50% of the current applied during galvanostatic charge cycle.

In certain embodiments, the discharge step can be a combination of highand low current density galvanostatic steps, allowing the cell chemistryto optimize coulombic efficiency and ensure maximum diffusion of activeions and its participation in electron-exchange reactions. Since thedischarge process is a function of the rate at which aluminum ionsdiffuse through manganese oxide, such a combination of high and lowcurrent prevents the build-up of localized charge at thecathode-electrolyte interface and optimizes the efficiency of the cell.

While not wishing to be bound by theory, it is believed that as smallregions at the cathode-electrolyte interface continue to build up chargeat relatively high current densities, the flow of ions gets impeded andthe reaction kinetics become slower. Therefore, a method that followssuch a high current draw with a short period of low current densitydischarge enables dissipation of this localized charge build-up, helpingthe ions diffuse through the surface and into the longitudinal depths ofthe cathode and as a result, freeing up the surface of the cathode forsubsequent ions to diffuse through to the bulk of cathode. It isbelieved that at higher current densities, aluminum ions do not havesufficient time to diffuse through the bulk of cathode, resulting in alocalized charge build-up at the cathode-electrolyte interface. As thehigh current density is momentarily replaced by a lower current density,aluminum ions begin diffusing through the bulk of the cathode, resultingin a reduction in the localized charge build-up at thecathode-electrolyte interface.

A typical voltage profile produced using such a discharge profileincorporating a combination of low-current and high-current pulses isshown in FIG. 18. FIG. 18 shows a discharge profile produced by acombination of low-current and high-current pulses. The battery wasassembled in a 2032 coin cell format and had an anode comprising analuminum foil treated with LiOH as described in Example 1, a cathodecomprising acid-treated lithium manganese oxide, a 0.5 M aluminumnitrate (aq) electrolyte and a 25 μm thick polypropylene separator withan average pore size of 0.067 μm. The current densities were switchedbetween 100 A/g (low-current pulse) and 500 A/g (high-current pulse),where the current is normalized with respect to the mass of the cathode.Similar approaches can be used with a system such as the one illustratedin FIG. 10 to improve the overall performance of the battery.

While the disclosure has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the disclosure. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the disclosure without departing fromthe essential scope thereof. Therefore, it is intended that thedisclosure not be limited to the particular embodiment disclosed as thebest mode contemplated for carrying out this disclosure, but that thedisclosure will include all embodiments falling within the scope of theappended claims.

1-29. (canceled)
 30. A method of operating a secondary batterycomprising (i) a first electrode, (ii) a second electrode, and (iii) anaqueous electrolyte disposed between the first electrode and the secondelectrode, the method comprising: causing transport of a polyatomic ioncomprising aluminum through the aqueous electrolyte between the firstelectrode and the second electrode.
 31. The method of claim 30,comprising charging the battery by causing transport of the polyatomicion comprising aluminum between the first electrode and the secondelectrode.
 32. The method of claim 30, comprising discharging thebattery by causing transport of the polyatomic ion comprising aluminumbetween the first electrode and the second electrode.
 33. The method ofclaim 30, wherein the polyatomic ion comprises one or more hydroxylgroups.
 34. The method of claim 30, wherein the polyatomic ion isAl(OH)₄ ¹⁻.
 35. The method of claim 30, wherein the aluminum in thepolyatomic ion is multivalent.
 36. The method of claim 35, wherein thealuminum in the polyatomic ion has valence of 3+.
 37. The method ofclaim 35, wherein the valence of the polyatomic ion during transport isless than the valence of the aluminum in the polyatomic ion.
 38. Themethod of claim 35, wherein the polyatomic ion is monovalent duringtransport.
 39. The method of claim 30, wherein the transport comprisesreacting hydroxides in the electrolyte with aluminum in the firstelectrode or in the second electrode to form Al(OH)₄ ¹⁻.
 40. The methodof claim 39, wherein the reacting occurs during charge or discharge ofthe battery.
 41. The method of claim 30, wherein the first electrodecomprises a manganese oxide.
 42. The method of claim 41, wherein thetransport comprises intercalating the polyatomic ion comprising aluminuminto the manganese oxide.
 43. The method of claim 41, wherein thetransport comprises deintercalating the polyatomic ion comprisingaluminum from the manganese oxide.
 44. The method of claim 41, whereinthe manganese oxide is a lithium manganese oxide and the method furthercomprises causing transport of an ion comprising lithium through theaqueous electrolyte between the first electrode and the second electrodewhile the polyatomic ion comprising aluminum is transported between thefirst electrode and the second electrode.
 45. The method of claim 44,wherein the lithium manganese oxide is an acid-treated lithium manganeseoxide.
 46. The method of claim 41, wherein the manganese oxide ismanganese dioxide.
 47. The method of claim 30, wherein the transportcauses the battery to discharge, wherein a discharge reaction during thedischarging is: Al(OH)₄ ¹⁻→Al(OH)₃+OH⁻+3e.
 48. The method of claim 30,wherein the transport causes the battery to charge, wherein a chargereaction during the charging is: Al(OH)₃+3e→Al³⁺+3e.
 49. The method ofclaim 30, wherein a porous separator physically separates the firstelectrode from the second electrode, the porous separator has an averagepore size of 0.067 μm to 1.2 μm, and the polyatomic ion comprisingaluminum transports through the porous separator during the transport.50. The method of claim 30, wherein the aqueous electrolyte comprises analuminum salt and a concentration of the aluminum salt in theelectrolyte is in a range from 0.05 M to 5 M.
 51. The method of claim50, wherein the aluminum salt is aluminum nitrate.